by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

Unseasonable warmth across the country this week likely has sealed the deal: 2012 will go down as the warmest year in U.S. history, according to data released Thursday by the National Climatic Data Center in Asheville, N.C.

So far this year, the USA has had a national average temperature of 57.1 degrees, 3.3 degrees above the long-term average and a full degree above the previous warmest January-November period, which was in 1934.

U.S. weather records date back to 1895. The warmest full year on record was 1998.

A total of 18 states in the central and northeastern USA have had record warmth this year and an additional 24 states are seeing a Top 10 warm year. No states are seeing a cooler-than-average year.

For 2012 not to be the warmest year on record, December would have to be 1 degree colder than any other December on record, and so far it's been unusually warm, 5-10 degrees above average, Crouch says.

"The first driver of the extreme warmth was March," reported Deke Arndt, chief of the climate monitoring branch of the climate center, last month. "March was the warmest March on record by far, and this caused 2012 to leap out way ahead of the pack."

"We had the warmest spring on record, the warmest July on record, and the third warmest summer on record. All of these together helped 2012 maintain a huge lead throughout the year," Arndt said.

Climate center data shows that 7 of the top 10 warmest years in U.S. history have occurred since 1998; Data from the environmental group Climate Central reports that average annual temperatures have been rising in every state since 1970, which they attribute to man-made global warming.

The heat also has kept much of the nation drier than average this year, helping contribute to the USA's worst drought since the 1950s and the second-worst wildfire season on record in acreage burned.

So far, three states -- Wyoming, Colorado and Nebraska -- are seeing their driest year on record.

The dryness has also caused a serious water shortage in both the Missouri and Mississippi Rivers, according to Arndt.

Mississippi River flow continued to decline, the U.S. Drought Monitor reported today, and it may be necessary to close parts of the river to barge and shipping traffic at some point. The monitor, a federal website that tracks drought, says that the U.S. Army Corps of Engineers is undertaking dredging and blasting operations in an effort to keep as much of the river navigable for as long as possible.

If the river is shut down, "this would slow the delivery for commodities, including fuel, and drive up prices for consumers," says Arndt.

Snow is also lacking so far this winter: Snow is covering only 7% of the USA now, the lowest percentage for this date since 2003, the National Weather Service reports. Chicago and Milwaukee are both on pace to set records for consecutive days without snow, if no snow falls in those cities within the next few days.